1. Field of the Invention
The present invention pertains to a wireless communication system including an access point connected via wireless communication to a backbone network such as a wired Local Area Network (LAN), and a system controller, an access point, a station, a communication control method, and a computer-readable medium storing a communication control program for use with the wireless communication system.
2. Description of the Related Art
In a general wireless communication system, an access point or a wireless base station is linked with a station or a wireless communication terminal in an infrastructure mode and the access point is coupled by wire with a backbone network such as a wired LAN.
In a recent communication technique, the access points are linked via wireless communication with each other to configure a backhaul communication line up to the backbone network using the wireless communication between the access points.
Some wireless communication systems associated with the present invention include first and second mobile communication terminals or stations as described in, for example, Japanese Patent Application Laid-Open No. 2004-320132 (document 1). In such systems, the first station detects at least intensity of a received radio wave from a wireless LAN access point or a communication state of the radio wave. If it is determined that communication is possible using a wireless LAN interface, the first station conducts communication by use of the wireless LAN interface. The second station operates in an infrastructure mode if it includes an access point function and operates in an ad hoc mode if it includes a client function.
In other wireless communication systems, if an instruction is issued and data to be communicated by the instruction is, for example, mobile picture data of which the amount is equal to or more than a predetermined amount of data, the system changes its operation mode to an infrastructure mode. Otherwise, the system changes its operation mode to an ad hoc mode as described in, for example, Japanese Patent Application Laid-Open No. 2004-349777 (document 2).
Referring now to FIG. 1, description will be given of problems of the general wireless communication system.
In the wireless communication system shown in FIG. 1, each of the access points 801a, 801b, and 801c includes two wireless interfaces 811 and 812.
The interface 812 is employed for a backhaul wireless line to relay communication between the access points. The interface 811 is adopted for access to accommodate a station 803a, 803b, 803c. 
The access interface 811 includes a function to arbitrate conflict between the stations according to a polling control operation and a function to conduct a power saving control operation for the stations. The backhaul interface 812 adopts in most cases wireless access control of the type based on a distributed coordinate function to implement a mesh configuration of the backhaul wireless link.
The access point 801 constructs, by use of one wireless interface, a backhaul wireless network of the backhaul line to relay communication between the access points. Therefore, it is required in the backhaul network that wireless communication is conducted by using one wireless channel shared in the overall network.
In the example shown in FIG. 1, to implement communication between an external network 805 and the station 803c accommodated in the access point 801c, it is required that the access point 801b relays the communication. However, since the access point 801b uses only one wireless interface for the backhaul network, a radio channel of one and the same frequency is required to be shared between backhaul links L812 and L813.
As a result, due to influences of, for example, a problem of a station exposed to wireless communication and a problem of a concealed station, it is difficult to secure scalability to expand the zone of the backhaul network and the communication quality at the same time.
Also, since the access link to accommodate the stations is constructed using one wireless interface, it is required that the plural stations accommodated by the access link carry out communication by sharing the radio channel. This hence leads to a problem that the number of stations to be accommodated by the access link is limited. Particularly, in a situation in which the stations are densely placed at a particular position, the influences of interference and access conflict are not negligible.
To solve the problems, if the user conducts a setting operation for the access points according to the state of the network such as the state of installation of the access points, the setting operation is troublesome and takes a long period of time.
To remove the problems, it can also be considered to construct the system by use of access points each of which includes many wireless interfaces. However, the number of channels available for the wireless LAN is limited. Therefore, even if the number of interfaces is desirably increased, the advantageous effect is limited. The desirable increase in the number of wireless interfaces makes it difficult to reduce the production cost.
Moreover, as in the communication system of FIG. 1, if the backhaul interface and the access interface are fixedly allocated to the wireless interfaces of the access stations, there likely appears depending on the topology and traffic of the network a case in which the traffic concentrates on a particular wireless link and hence the network performance is lowered due to congestion. Contrarily, there may exist a wireless interface not used. Particularly, in use of a station of high mobility, these problems conspicuously take place when the traffic varies and/or moves.
According to the technique described in Japanese Patent Application Laid-Open No. 2004-320132, when a station relays wireless communication, the infrastructure mode and the ad hoc mode are discriminately employed according to the function of the station. That is, consideration is given neither to the operation of the access point to control connection of stations belonging thereto nor to the operation in which the setting of the destination of connection in the wireless interface is changed according to the state of connection to the backhaul wireless line for wireless communication between access points or is changed to the access use to accommodate the station.
According to the technique described in Japanese Patent Application Laid-Open No. 2004-349777, to adapt to the communication traffic predictable by an instruction thus issued, the setting of connection of the wireless interface is changed to the infrastructure mode or the ad hoc mode. Therefore, consideration is given neither to an operation of the access point to control connection of stations belonging thereto nor to the operation in which the setting of the destination of connection in the wireless interface is changed according to the state of connection to the backhaul wireless line to conduct wireless communication between access points or is changed to the access use to accommodate the station.